A fluorescent tube is made in its conventional form from a hermetically sealed glass tube filled with low-pressure mercury vapour and with a rare gas such as neon, argon or krypton. Electrodes inside the tube, when in operation, emit electrons that excite the gas mixture inside the tube and lead to emissions in the ultraviolet range (for example at about 300 nm).
This ultraviolet light is converted into visible light by means of a fluorescent coating deposited on the inside of the tube.
In the case of a “monolayer” coating, the coating comprises luminophore particles, known, for example, under the names BAM, CAT or YOx, and also alumina particles that act as reflectors.
Generally, 80% of this layer is composed of luminophore particles and 20% of particles of alumina or of alumina of gamma type.
The luminophore particles generally have a size d50 of between 4 μm and 10 μm.
Now, it is known that the cost of luminophores is predominant in the overall cost of the coating.
In a thesis defended on 17 Oct. 2008 in the University of Paris 6 by Serge Itjoko, a study was undertaken firstly to model the behaviour of fluorescent layers and secondly to identify optimization routes in terms of yield and cost. This thesis is cited by reference in the present patent application as a prior art.
It emerges in particular from this study of a mixed layer or monolayer that an optimization may be achieved by “selecting luminophore radii that are much smaller than those of the existing luminophores, i.e. radii of between 0.4 μm and 1.2 μm, and radii of alumina grains that are much larger than those of the existing alumina grains, i.e. radii of greater than 0.6 μm”.
This study gives merely a theoretical result, given that it is a theoretical modelling study, but gives no indication how such luminophores and alumina particles may be obtained. In particular, on page 173 of this thesis, it is stated that “commercial luminophores have a radius ranging between 3 μm and 6 μm” and that luminophores with a smaller size than this have not yet been developed.